This proposal is an attempt to broaden our knowledge of the nature of eukaryotic gene regulation at a molecular level. The coordinate regulation of gene activity is essebtuak for normal cellular function. Various types of control mechanisms have evolved to ensure that only proteins needed for the specific cellular environment at any given time are synthesized. In an attempt to elucidate some of the control mechanisms involved, we propose to study the histidine biosynthetic pathway in the yeast Saccharomyces cerevisiae. Both genetic and biochemical approaches will be utilized. Our general plan will be to saturate this pathway with many different dypes of mutations and to analyze the molecular perturbating which result. Mutations which cause either derepression or repression of the histidine genes by altering either specific or general control mechanisms will be isolated and characterized. In vivo and in vitro generated lesions resulting in base substitutions, deletions, and insertions will be analyzed: assigned to complementation groups, mapped, tested for dominance relationships, analyzed for mRNA and protein levels, cloned, and sequenced. The possible role of transposable elements in gene regulation will also be explored.